Voice and data generally is transmitted between communications devices as either electrical signals through electrical waveguides, free space, or as optical signals through optical fiber.
One way to provide optical transmitters with greater transmission range and/or with greater robustness to band-limited channels is to encode the data into the signal using a well-known method called duobinary encoding [A. Lender, The Duobinary Technique for High-Speed Data Transmission, IEEE Trans. Commun. Electron. 82, 214-218, 1963]. A signal having duobinary encoded data typically has 2-3 times better dispersion tolerance than an equivalent signal in which the data is coded as conventional binary. However, there is a price to pay. Duobinary signals perform worse than conventional binary signals when the dispersion is very low, such as when the data is transmitted over a short transmission distance or the transmission medium is extremely well dispersion-compensated. This is not only a problem for well-compensated systems, but also in connection with testing and verification of network components since testing and verification usually involves placing the network components in a back-to-back mode, involving short transmission distances. An ideal broadband optical or electrical data transmitter would have good transmission characteristics for both long transmission distances and in back-to-back operation, i.e., short or well-compensated transmission situations.